I have always stopped worrying about validation with forums anyways. Its probably less of a problem with Xenforo and the lack of certain support in the editor. But user input on other forum software is always going to have the chance of killing clean validation.
CSS 3 validation might also be silly. The warnings might show for anything deprecated from CSS 1/2 which the majority of CSS on the site won't be from 3. I try to use as little CSS3 as I can to provide a consistent look across browsers because the truth is some of the drafts are still only a couple years old and not widely supported.
Looking at your link now most of the warnings are from VERY GOOD practices:
Property -moz-box-sizing is an unknown vendor extension
Using vendor specific extensions to support newer CSS that was implemented in older versions of some browsers is a very good thing in my book if not tedious. Its a warning because the like the validator the browser will go "I don't know what that is" and skip it. Time wasted parsing but no harm done.
Alot of the CSS errors are tripping on microsoft specific stuff.